Very small, or micromachined openings in opaque workpieces such as aluminum, stainless steel, or plastics can create surfaces that appear uninterrupted to unaided human vision. However, when light is impressed upon opposite surfaces, light appears to be coming directly from the apparently uninterrupted surfaces. It is necessary for these openings to be formed very precisely to achieve all desirable characteristics of the resulting surface including smoothness of the surface, adequate transmissibility of light, and adequately retained strength and integrity. Micromachined openings are also useful in allowing other electromagnetic fields and fluids to pass through workpieces, whether optically opaque or otherwise while avoiding degrading various desirable characteristics of the workpieces and associated surfaces.
Workpieces to be drilled in this manner comprise a variety of materials, including metals, including aluminum, various types of plastics, epoxies, and composite materials such as fiberglass or various types of clad substrates. Tapered holes can be drilled in all of these different materials, although laser pulse parameters, such as focal plane location, pulse energy, pulse duration, pulse temporal profile, pulse repetition rate, number of pulses, spot size or wavelength may have to be altered to for a particular type of workpiece.
FIG. 1 shows a schematic diagram of a prior art laser drilling system 1. Laser drilling systems typically comprise a laser 10 emitting laser pulses along a laser beam axis 12, beam shaping optics 14, beam steering optics 16, scan lens 18, a controller 20, and a stage 22 with motion control devices (not shown) for holding the workpiece 24 and moving it in relation to the laser beam axis in up to six axes which include translation in three orthogonal axes (X, Y and Z) and rotation about the three axes (ρ, φ and θ). Note that the workpiece or the laser beam axis or both may be moved along or rotated about any of these axes to achieve this relative motion. The controller 20 directs the laser to emit pulses along the laser beam path and then coordinates the motions of the beam steering optics and the stage to position the workpiece to cause the laser beam path and hence the laser pulses to intercept the workpiece at the desired point at the desired time.
Hole taper is defined as the ratio of the top diameter of the hole, which is the diameter of hole as measured at the surface of the workpiece first impinged by the laser beam, to the exit diameter, which is the diameter of the hole as measured at the surface where the laser beam exits the workpiece following drilling. Taper is typically introduced into the hole by adjusting power so that only a small fraction of the hole volume is removed by each pulse and then programming the system to move the laser beam path in a decreasing spiral pattern as the laser is pulsed. Moving the laser beam path in a decreasing spiral or successively decreasing radius circular pattern, called trepanning, can drill a hole which tapers from a larger entrance diameter to a smaller exit diameter. This is made more difficult by high aspect ratio holes where the ratio of depth to diameter is greater than three to one.
FIG. 2 shows a schematic cross-sectional view of a prior art hole drilled by the trepanning or spiral methods. FIG. 2 shows the workpiece 30, the top diameter 32, the exit diameter 34 and the stair stepped side walls 36. Not shown in this schematic drawing is the rough nature of the surface finish on the stair stepped sidewalls. When a laser pulse removes material from the hole, the material is ejected from the location where the pulse strikes the workpiece in gaseous, liquid and possibly solid form. As the laser beam, which is focused into a much smaller spot size than the diameter of the hole, is moved to another location to deliver the next pulses, the locations already drilled have a chance to cool. This permits the material ejected by subsequent pulses to cool and stick to the previously drilled surfaces, causing a roughened surface. The stair-stepped side walls and the rough surface finish combine to yield a hole that transmits light unevenly. Holes drilled in this manner will present differing appearances with the uneven sidewalls reducing the total amount of light and causing each hole to transmit light unevenly, thereby causing a pattern of holes to present an uneven appearance. This effect will also cause the appearance of the lit hole to change unevenly with varying viewing angles.
One reason this problem exists is that in order to meet the requirements for precision and throughput, existing systems designed for drilling vias in circuit boards are used to drill these holes. An exemplary system of this type is the Model ICP5650 laser drill system manufactured by Electro Scientific Industries, Inc., Portland, Oreg. These systems are typically designed to drill small blind vias in circuit board materials with UV lasers using the trepanning methods described above. While these systems have the reliability and power to drill tapered holes with the desired precision and throughput, the final product is less than desirable.
Thus, two problems exist when drilling tapered holes in this fashion. The first is that many pulses are required to drill a hole in this manner. The speed with which holes can be drilled is a function of both pulse repetition rate and the speed with which the laser beam path can be accurately moved from point to point. These factors limit the speed with which a taper hole can be drilled and hence the throughput of the system. The second is that drilling a tapered hole in this fashion leaves the interior surface of the hole rough and uneven as a result of being formed by successive laser pulses. Although the desired exit hole diameter and location of the holes can be achieved fairly accurately with this method, the uneven finish on the side walls causes the holes to vary in appearance, with some holes appearing lighter or darker than others. In addition, the uneven geometry of the side walls causes the holes to change in appearance with changes in viewing angle, another very undesirable effect.
Accordingly, there is a continuing need for an apparatus for laser drilling tapered holes in substrates, capable of forming relatively smooth, high quality holes with predictable exit hole diameters, while maintaining improved system throughput.